A lot of the most basic traits we associate with humans, not the least of which include intelligence and walking on two legs, evolved when our ancient habitat changed from dense forest to wide-open grassland. So what caused this change?

The reason behind East Africa's big shift from forest to grassland has puzzled scientists, with lots of different explanations being advanced to account for this crucial change. The North Atlantic might have cooled, the amount of carbon dioxide in the atmosphere might have decreased, or more volcanoes might have erupted. All could well be pieces of the puzzle, but they're not the entire explanation - none of these would be big enough to be responsible for the whole shift.

Now, Peter deMenocal of Columbia's Lamont-Doherty Earth Observatory says that he has the answer, which he recently presented at the annual meeting of the American Association for the Advancement of Science. He says the temperature gradients of the Indian and Pacific Oceans - in other words, how the temperatures of the oceans change over a given distance - can explain why East Africa underwent its crucial makeover.

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Until about two million years ago, deMenocal says that the Indian Ocean had a uniform temperature of about 27-28° Celsius. This meant there was plenty of warm water off the coast of East Africa in the Arabian Sea, and this would have provided the necessary humidity and rainfall to sustain heavy forestland. But deMenocal's research indicates that this began to change dramatically at two million years ago, as the Arabian Sea dipped to 25°C, while the more eastern parts of the Indian Ocean rose slightly to 28-29°C.

According to the researchers' climate models, this shift to what is still the defining temperature gradient of the Indian Ocean would have stolen a huge amount of available warm water away from East Africa, and rainfall would have crashed by up to 30%. This would have been more than enough to cause the emergence of modern savannas, and the timing seems to line up absolutely perfectly.

Of course, this just explains why the grasslands emerged. What it doesn't explain is why the temperature gradient of the Indian Ocean - not to mention a similar shift away from uniform temperatures in the Pacific - happened two million years ago. Something even bigger and more fundamental must have happened to cause those shifts, and now scientists need to locate the next link in this chain.